1. Field of the Invention
This invention is concerned with a one-step method for preparing vanadium-phosphorus catalysts with high activity and selectivity and good physical strength for the oxidation of alkenes, alkanes, cycloalkanes and mixtures rich therein to dicarboxylic acid anhydrides, particularly butane or butene to maleic anhydride.
2. Description of the Prior Art
Vanadium-phosphorus complex catalysts for the oxidation of butene to maleic anhydride, are described in U.S. Pat. No. 3,293,268. Such catalysts operate at temperatures greater than 500.degree.C. In general, yields of maleic anhydride with such catalysts are relatively low and not commercially attractive or feasible.
Metal-promoted vanadium-phosphorus complex catalysts are described in U.S. Pat. No. 3,156,705. The metal promoters, identified as phosphorus stabilizers, are broadly disclosed to include transition metals and rare earth metals. The catalysts are taught for oxidizing an olefin (butene) to a dicarboxylic acid anhydride (maleic anhydride). Further, the catalysts described in this patent involve the use of a closed reflux system during their preparation, as compared to the simple open system used in this invention.
In application Ser No. 261.030, filed June 8, 1972, and continuation-in-part thereof Ser. Nos. 379,667 and 461,777 filed, respectively, July 16, 1973 and Apr. 17, 1974 there is described an improved process for oxidizing an alkane to a dicarboxylic acid anhydride in the presence of a catalyst comprising a complex reaction product of a vanadium oxysalt and phosphoric acid promoted with one or more Cr, Fe, Hf, Zr, La and Ce. The atomic ratio of P/V is between about 0.5 and about 2, and the atomic ratio of promoter metal/V is between about 0.0025 and about 1, in such catalysts. Ser. Nos. 261,030 and 379,667 are now abandoned and Ser. No. 461,777 is now U.S. Pat. No. 3,888,886.
As an improvement over, and an extension of, the catalysts described in said applications and in U.S. Pat. Nos. 3,156,705 and 3,293,268 the present invention is concerned with a simple, one-step method for making V-P catalysts. In prior catalyst preparation methods using HCl, at least 5 moles or more of HCl were required per gram atom of vanadium. Such preparation methods present a corrosivity problem requiring the use of expensive corrosion resistant equipment. They also increase the volume of material that must be handled and involve the use of closed reflux systems. The method of this invention, however, does not involve the use of hydrogen halide (HCl) and eliminates problems associated with its use. It also eliminates the need to handle large volumes of liquid and the use of a closed reflux system. When the vanadium source is a halogen-containing vanadium salt, the amount of by-product hydrogen halide encountered is comparatively small and involves a relatively negligible problem.